Robotic manipulators may be divided into two main categories, parallel and serial manipulators. Serial manipulators, which are more common in the industry, have several links in series usually connected by rotary or sliding joints. They are analogous to the human arm which has a series of links hinged at the shoulder, elbow and wrist. The configuration of serial manipulators necessitates the location of the driving motors to be at the joints themselves or the use of a heavy or complicated linkage for transferring the motion from the base of the robot to the joints. This is a disadvantage since it requires the movement of the large mass of the manipulator and drives even for a small payload. Further, the positional error of the end effector of a serial manipulator is the accumulation of the errors in the individual links so that by increasing the size or number of links the error associated with the position of the end effector increases.
In contrast to serial manipulators, the links of a parallel manipulator function in parallel to determine the movement of the end effector. A flight simulator and camera tripod are two examples of this kind of mechanisms. If one of the legs of a tripod is extended or moved, it changes the position of the end point. Parallel manipulators have relatively lower mass to payload ratio since the links work together and the actuators are mounted on a stationary base. They also have better precision since the error in the end effector is in the same order of actuators' error.
Low inertia, and therefore, high speed manipulation is one of the main applications of parallel robots. U.S. Pat. No. 4,976,5821 issued to Clavel, entitled ‘Device for the Movement of and Positioning of an Element in Space’, and reported further in Clavel, ‘Delta, a Fast Robot with Parallel Geometry’, Proceeding of International Symposium on Industrial Robots, pp. 91–100, Apr. 1988, discloses one of the most successful mechanisms of this kind which produces movement with three pure translational degrees of freedom at its end effector. In this manipulator of Clavel, rotating arms are connected to the end effector using three parallelograms. The parallelograms constrain the end effector to be parallel to the base plate at all times and therefore, three pure translational movements are achieved.
Other manipulator designs such as disclosed in L-W. Tsai, ‘Kinematic of a Three-DOF Platform With Extensible Limbs’, Proceeding of the Conference of Recent Advances in Robot Kinematics, pp. 401–410,1996, also provide pure translational movement of the end effector with three translational degrees of freedom. In the Tsai mechanism, three linear actuators connect the end effector to the stationary platform with universal joints. The specific configuration of the universal joints guarantees the three translational motions of the end effector.
There are also parallel mechanism robots with 6-DOF such as the hexa pod, see Griffis M., Crane C., et Duffy J., ‘A smart kinestatic interactive platform’, In ARK, pp. 459–464, Ljubljana, 4–6 July 1994, and the hexa robot disclosed in U.S. Pat. No. 5,333,514 issued to Toyama et al. entitled ‘Parallel Robot’.
In general, parallel mechanism robots have higher stiffness to weight ratio, moment and torque capacity, and better accuracy. They also benefit from a simpler mechanism due to the elimination of drive trains and, also lower moving mass due to the stationary location of the actuators. Further reduction in the moving inertia of parallel mechanisms may be achieved by replacing the rigid links with tensile means such as cables. Replacing the rigid arms not only reduces the moving inertia but it lowers manufacturing cost and simplifies the mechanism structure by eliminating many joints.
Using cables in cranes such as disclosed in U.S. Pat. No. 3,286,851 issued to J. R. Sperg entitled ‘Cargo Handling Rig’, and similar applications, see U.S. Pat. No. 5,967,72910 issued to G. F. Foes entitled ‘Bottom Discharge Rotating Ring Drive Silo Unloader’, is older than robotics, however in recent years several attempts have been made to design cable actuated manipulators. Some of these manipulators are designed to imitate human arms and can be considered as serial manipulators with parallel actuators, see U.S. Pat. No. 3,631,737 issued to F. E. Wells entitled ‘Remote Control Manipulator for Zero Gravity Environment’; U.S. Pat. No. 3,497,083 issued to V. C. Anderson, R. C. Horn entitled ‘Tensor Arm Manipulator’; and U.S. Pat. No. 4,683,773 issued to G. Diamond entitled ‘Robotic Device’.
A pure parallel cable actuated mechanism is disclosed in S. Kawamura, W. Choe, S. Tanaka, S. R. Pandian, ‘Development of an ultrahigh Speed Robot FALCON using Wire Drive System’, Proceeding of IEEE Conference on Robotics and Automation, pp. 215–220, 1995. This manipulator has seven active cables to provide 6-DOF for the end effector. This mechanism does not have any rigid link in its structure and the cables are extended in both sides to maintain tension in the cables.
U.S. Pat. No. 4,666,362 issued to S. E. Landsberger and T. B. Sheridan entitled ‘Parallel Link Manipulator’ discloses a manipulator which uses six active cables and a passive collapsible link. The collapsible link applies a pushing force between the moving and stationary platforms in order to keep all cables in tension.
U.S. Pat. No. 5,313,854 issued to H. A. Akeel entitled ‘Light Weight Robot Mechanism’, discloses another combined cable-collapsible mechanism which moves the end point of the collapsible shaft in the space but does not have any control on its orientation.